The field of the invention pertains to means for delaying acoustic signals generated by electroacoustic drivers or transducers in sound reproduction speakers. In particular, the invention pertains to means for assuring the simultaneous receipt of sounds by the listener generated by simultaneous electric signals input to the speaker.
Typically, the electroacoustic drivers are mounted in apertures in a baffle forming a portion of the speaker cabinet. Unfortunately, electric signals impressed upon two or more of the drivers simultaneously will not create a simultaneous acoustic signal at the position of the listener. In practice the low frequency filter and low frequency driver exhibit more delay than the high frequency filter and high frequency driver. Therefore, prior art efforts involve adding delay to the air path of the high frequency driver. Air path delay techniques are disclosed in U.S. Pat. Nos. 3,824,343 and 3,927,261 wherein a plurality of drivers are physically positioned to provide increased air paths in proportion to the rise times of the drivers. Similarly, U.S. Pat. No. 4,015,089 discloses a plurality of drivers arranged in staggered relation along their radiating axis at a predetermined spacing from each other. A second approach to delay, disclosed in the prior art, pertains to active electrical network means. U.S. Pat. No. 4,151,369 discloses a decoder, gain and delay system for multiple speakers in a "surround sound" configuration. The speakers are located circumferentially about the listener at various radii.
Another approach to active networks is disclosed in a series of articles by Siegfried Linkwitz entitled "Loudspeaker System Design" published in WIRELESS WORLD, May and Jun. 1978 issues. Linkwitz describes active crossover networks with active delay compensation. He comments that the design flexibility of active networks outweighs the cost saving of a passive network; however, no passive network is disclosed. He further emphasizes at the end of his series of articles that computer optimization would be required to design a passive network comparable to his active crossover networks.
Japanese Patent Publication No. 54-13321 discloses the combination of a "phase delay" circuit in the low frequency filter circuit with the high frequency driver positioned behind the low frequency driver. Since the addition of additional electric delay in the low frequency filter circuit further increases the time differential between the arrival of high frequency sound and low frequency sound at the listening point, the high frequency driver must be located further behind the low frequency driver than would otherwise be necessary. Such an electrical configuration does not lend itself to applications wherein both drivers are in the same baffle (i.e., not behind one another) or co-axial drivers wherein the high frequency driver is in front of the low frequency driver. Japanese Patent Publication No. 52-33517 discloses the application of a "phase delay" circuit in the high frequency filter circuit in addition to or in substitution for placing the high frequency driver behind the low frequency driver. The object of this disclosure as well as the other above Japanese disclosure is to flatten the phase response of the combined electroacoustic network at the location of the test microphone in front of the loudspeaker.
Merely flattening the phase response of a combination of drivers in addition to the amplitude response by geometrically moving the high frequency driver behind the low frequency driver or substituting a delay circuit for the geometric movement and adjustment of the relative polarities however is not sufficient to remove coloration and amplitude peaking over a large included volume in front of the loudspeaker.
In particular, neither Japanese disclosure is directed to co-axial drivers wherein the high frequency driver is in front of the low frequency driver. Nor is Publication No. 52-33517 directed to overcoming the phase response changes in the low frequency driver near the amplitude response band edge.
In a paper entitled "Is Phase Linearization of Loudspeaker Crossover Networks Possible By Time Offset and Equalization?", by John Vanderkooy and Stanley P. Lipshitz, presented at the 70th Convention of the Audio Engineering Society (1981, Preprint 1857 (C-6)), the authors emphasize that phase linearity and perfect impulse response are not possible without the use of complicated delay equalizers (page 2). The paper emphasizes that adding delay to any of a variety of conventional filter-driver networks cannot achieve phase linearization.